The population of Japan is currently aging at a progressively faster rate, and the number of patients with dementia such as Alzheimer's disease is correspondingly increasing. The number of dementia patients at present is estimated to be approximately 4,500,000 people and it is predicted that the number of patients will continue to increase with aging in future. The care of such dementia patients involves large economic burden as well. The establishment of an effective therapy as soon as possible is urgently sought.
The three pathological features: (1) loss of nerve cells, (2) formation of plaque amyloid deposits (senile plaque), and (3) accumulation of fiber aggregates in nerve cells (neurofibrillary tangle) are commonly observed in patients with Alzheimer's disease, and these are causes of cognitive impairment (Alzheimer-type dementia), which is a clinical manifestation of Alzheimer's disease. Treatments of Alzheimer-type dementia currently conducted are symptomatic treatments which temporarily ameliorate symptoms of the dementia using a choline esterase inhibitor, an NMDA inhibitor, and the like. However, their effects are very limited and the establishment of a causal therapy that suppresses development and progression of Alzheimer's disease is strongly sought.
The establishment of a causal therapy of Alzheimer's disease requires elucidation of the pathogenic mechanism of Alzheimer's disease. Known Alzheimer's disease includes familial (hereditary) Alzheimer's disease, which is caused by genetic factors, and sporadic Alzheimer's disease, which has no genetic history. Currently, causative genes and risk factors of familial Alzheimer's disease are being revealed. Since pathological findings and clinical manifestations are common between familial Alzheimer's disease and sporadic Alzheimer's disease, it is predicted that a common mechanism exists in the process of disease onset. It is hoped that the study of the pathogenic mechanism of familial Alzheimer's disease leads to elucidation of the onset mechanism of sporadic Alzheimer's disease.
One of the causative genes of familial Alzheimer's disease is a gene encoding amyloid precursor protein (hereinafter, referred to as APP). APP is cleaved by β-secretase and γ-secretase to produce amyloid β (hereinafter, referred to as Aβ). There are two different Aβs that differ in the cleavage point in APP: Aβ40 consisting of 40 amino acids, and Aβ42 in which 2 amino acids are added to the C-terminus of Aβ40. Whereas Aβ40 is dominant in physiological amount of production, more Aβ42 is contained in senile plaques. Furthermore, it has been revealed that mutated APPs increase the expression ratio of Aβ42 and that Aβ42 is easier to agglutinate in comparison with Aβ40. From these facts, the following theory is proposed: Aβ42 first aggregates and, having them as nuclei, Aβ40 aggregates and accumulates, the formation of amyloid fibrils thereby progress, and Alzheimer's disease develops.
It has been found that the aggregation of Aβ in the brain starts with binding of Aβ to GM1 ganglioside (hereinafter, referred to as GM1), constituting the nerve cell membrane, to form a complex (GM1-bound Aβ: hereinafter referred to as GAβ) (Non-Patent Documents 1 and 2). Moreover, it has been confirmed by the analysis using vesicles of artificial lipid membrane that GM1 assembles in the nerve cell membrane in a cholesterol concentration-dependent manner to form clusters and Aβ forms GAβ by specifically binding to these GM1 clusters (Non-Patent Document 3). Furthermore, it has been confirmed by the analysis using antibodies that specifically recognize GAβ that GAβ has a structure different from that of soluble Aβ (Non-Patent Document 4). From these findings, a hypothesis was proposed that when Aβ binds to GM1 clusters, it changes into GAβ having a different structure and this GAβ serves as “seeds” and promotes the aggregation of Aβ and a large number of research results that support this hypothesis have been obtained subsequently (Non-Patent Documents 5 to 9).
Various compounds have been developed and tested in clinical trials so far for developing a method for radical treatment/prevention of Alzheimer's disease. Examples of such conventionally well-known compounds include inhibitors of β-secretase, which cleaves at the N-terminal side of Aβ when Aβ is generated, and inhibitors of γ-secretase, which cleaves at the C-terminal side of Aβ (Patent Document 1, Non-Patent Document 10). These inhibitors have been expected to be capable of suppressing formation of amyloid fibrils by inhibiting the production of Aβ. However, APP is not the only specific substrate of 1-secretase or γ-secretase. As a result, the γ-secretase inhibitors, in particular, have the problem of serious side effects due to the inhibition of Notch signals deeply involved in development and differentiation and also due to the inhibition of the physiological metabolism of APP itself. Also, a large number of low molecular weight compounds that inhibit polymerization of Aβ have been developed (Patent Documents 2 to 6). These compounds prevent the elongation of amyloid fibrils or divide the fibrils by inhibiting the polymerization of Aβ. However, it is suggested that polymerization of Aβ is restarted if the administration of the compound is stopped and there may be the possibility that the division of fibrils itself generates new seeds of polymerization and a sufficient suppressant effect has not been obtained on the formation of amyloid fibrils.
Meanwhile, a method for suppressing amyloid fibril formation using an antibody (4396C antibody) that specifically binds to GAβ has been reported (Patent Document 7). The 4396C antibody radically suppresses the formation of amyloid fibrils by specifically recognizing and binding to GAβ and inhibiting the Aβ polymerization with GAβ. However, there are problems in that production of an antibody is time consuming and costly. Moreover, agents containing an antibody have the problem that the mode of their administration is limited and clinical use thereof is difficult because there is a problem in delivery to the brain.